1. Technical Field
The disclosure relates to the field of cement plugs in oil and gas wellbores. More particularly, the present invention relates to an improved system for identifying the location of a cement plug and the like within a wellbore.
After drilling a hole into a desired location, a casing is inserted into the wellbore to stabilize the structure of the wellbore. Cementing is further required to adequately support the casing, provide zone isolation and prevent mixing of fluids. The process of cementing is well known in the art. After insertion of the casing into the wellbore, the casing is filled with drilling fluid or mud (hereinafter referred to as “drilling fluid”). A bottom cement plug containing a rupturable disk or diaphragm is then inserted into the casing. The bottom cement plug may also be referred to as a displacement plug. Cement slurry is pumped on top of the bottom plug to move the plug downwards and to displace the drilling fluid out of the casing and into the annulus between the casing and the wellbore rock. A top cement plug is then positioned on top of the cement slurry and additional drilling fluid is pumped into the casing to move the top cement plug, the cement slurry, and the bottom cement plug through the casing. Float equipment at the bottom of the casing prevents the bottom cement plug from further movement upon contact. With the combination of the continuous pumping of drilling fluid, this causes a build-up of pressure sufficient to breach the rupture disk within the bottom cement plug.
When the rupture disk is breached, the cement slurry moves through the bottom cement plug, the bottom end of the casing, and into the annulus between the casing and the wellbore rock. The top cement plug follows the cement slurry until it is stopped by the float equipment at the bottom of the casing. The subsequent pressure increase indicates that the top cement plug has reached the bottom of the casing and for the operating unit or personnel to cease pumping of the drilling fluid, thus ending the cementing operation.
Optimal cementing jobs rely on accurate identification of the location of the cement plugs. Cementing operations currently rely on volumetric displacement calculations to determine the location of the cement plugs. However, this method suffers from low accuracy due to factors including long casing strings, large diameter casing, and variable diameter within casings. Accurate identification of the location of the bottom cement plug is important to prevent over- and underdisplacement of the cement. Overdisplacement occurs when all the cement slurry is moved outside the casing and may result a cement deficiency around the bottom of the casing. Underdisplacement leaves cement within the casing which needs to be later removed. Both over- and underdisplacement require remedial operations which are often expensive and time consuming.
For reference to an existing description of cement plug location systems please see U.S. Pat. No. 2,999,557 “Acoustic Detecting and Locating Apparatus” (Smith), U.S. Pat. No. 4,468,967 “Acoustic Plug Release Indicator” (Carter), U.S. Pat. No. 6,585,042 “Cementing Plug Location System” (Summers), U.S. Pat. No. 6,634,425 “Instrumented Cementing Plug and System” (King), and U.S. Pat. No. 7,013,989 “Acoustical Telemetry” (Hammond) the disclosures of which are hereby incorporated by reference.
Prior disclosures of cement plug location systems, such as the patents described above, are not practical in an industrial setting, thus prompting a need for an improved system. Moreover, there is scant evidence that preexisting cement plug location systems are effective at the scale needed, or that they are used commercially in any significant measure. Some examples of such prior systems include: systems that rely on signals reflected over great distances; systems that rely on measuring hard wiring or cable, or using the wire or cable to transmit a signal; or systems which use a dual telemetry system. These prior systems suffer from problems such as: significant signal attenuation, cost inefficiency and/or physical impossibility at drill sites. As such, modern oil well drilling operations continue to use volumetric displacement calculations to determine the cement plug location, instead of implementing the aforementioned systems.
A need exists for an improved cement plug location system having increased accuracy and efficiency in a wellbore.